Research

Plants growing on unusual geologies and other extreme substrates are model organisms for the study of ecology and evolution. My primary research focuses on using such plants and their habitats to explore questions on the role ecology plays in the evolution of individual plant species and the assembly of plant communities.

I am also interested in the effects of heavy metal contamination on plants and other biota, specifically research relating to metal transfer from contaminated soil via plants to other biota. Related areas of research include 1) phytoremediation, the use of plants to clean up metalcontaminated soil, and 2) the impacts of atmospheric deposition of metals and nutrients on native and invasive plant interactions. Additional interests include tropical plant ecology & conservation and ethnobotany.

Streptanthus polygaloides is a serpentine endemic found only in the western slopes of the Sierra Nevada in California. It can hyperaccumulate Ni (>0.1% of dry leaf tissue) and is one of only two Ni hyperaccumulators confirmed from continental North America. Its ability to hyperaccumulate Ni makes it a candidate for phytoremediation and phytomining. Further, a plant bug, Melanotrichus boydi, is endemic to and monophagous on S. polygaloides. Recent studies have shown S. polygaloides consists of four sepal color morphs differing in their Ni accumulation. Two of these morphs are restricted geographically, being found in only one or two counties. We aim to better characterize these morphotypes in terms of ecology, ion accumulation, taxonomically important traits, and inter-morph reproductive compatibility. If morphotypes are ecologically distinct, differ in taxonomically important traits, and show reduced reproductive compatibility, they may be worthy of both taxonomic subdivision and—depending on the abundance of each taxon—for subsequent listing for conservation. The work is done in collaboration with Mike Fong (MA 2011, SJSU), Nate Pope (UC Davis now UT Austin), and Dr. Robert Boyd, Department of Biological Sciences, Auburn University, Alabama. See recent publication.

In collaboration with Janis Strommen (SJSU, undergraduate) and Dr. Jeff Honda, we are also studying the life history and ecology of Melanotrichus boydi, an insect specializing on Streptanthus polygaloides. For additional research on M. boydi see here.

Lasthenia californica complex has provided a model to explore the role soil conditions play in the radiation of races (A and C) and species (L. californica and L. gracilis) within closely related taxa (See Bohm and Rajakaruna 2006). Lasthenia californica (race A) and L. gracilis (race C) are restricted to edaphically distinct regions within the serpentine outcrop at Jasper Ridge Biological Preserve, Stanford University. The purpose of this study is, first, to show the two taxa at Jasper Ridge are locally adapted to distinct soil conditions within the serpentine outcrop. Second, to determine if the distinct boundary between the taxa is present throughout the growing season (i.e. from time of germination), or if it only exists at the time of flowering. Thus, two main hypotheses will be tested. First, that both taxa will show higher fitness in their native localities within the outcrop. This hypothesis will be tested by conducting a reciprocal transplant experiment and using standard selection gradient analysis. Second, the taxa are found throughout the outcrop at the beginning of the season (i.e. at time of germination), then as the season progresses, changes in soil moisture (and other corresponding changes in soil chemistry) will lead to the distinct racial boundary previously documented at the time of flowering (see Rajakaruna and Bohm, 1999). This second hypothesis will be tested using DNA analysis of individual plants collected throughout the season as per genotyping methods developed by Dr. Kathleen Kay, University of California, Santa Cruz. At time of genotyping individual plants, soil physical and chemical information will also be recorded to determine if distributional changes of taxa correspond to seasonal changes in edaphic features. The work is done in collaboration with Teri Barry (MS Candidate, SJSU), Dr. Kathleen Kay and Jenn Yost. See recent publication.

Additional Ongoing Research

Is postfire recruitment of chaparral shrubs constrained by local adaptation to soils (serpentine vs sandstone) or microclimates (north vs south slopes)?

Fire plays a key role in the ecology of chaparral, or evergreen shrublands, an ecosystem unique to the five Mediterranean-climate regions of the world including California. Seeds of many chaparral plants germinate only in the year after a fire, and so the age, reproduction, dispersal potential, and community composition of chaparral plants are controlled by the infrequent occurrence of fire, typically once every 1-10 decades. Prescribed burns are seldom possible in Californian chaparral. What research has been done on the fire ecology of this ecosystem has been largely opportunistic, taking advantage of naturally occurring fires. More than 5,000 hectares of accessible public lands burned in 2008, creating a rare opportunity to examine whether three dominant chaparral shrub species (Ceanothus cuneatus, Adenostoma fasciculatum, Eriodictyon californicum) are locally adapted to the sites on which they are found. Tests will be performed to determine if regeneration occurs only from seed produced on particular soil types (sandstone or serpentine) or on particular slope aspects (warm south-facing or cool north-facing), or whether seed produced in a particular habitat can successfully germinate and establish in other soil types or on slopes with different aspects. Results from this study will provide additional information for science-based management of chaparral ecosystems. This information will contribute to both the restoration of degraded chaparral and to the ability of resource managers to predict and plan for shifts in species distributions in response to climate change. The work is done in collaboration with Dr. Susan Harrison, Annette Bieger, and Jamie Horvath. See recent publication.

The role of adaptive evolution versus ecological sorting processes in the formation of serpentine chaparral in california

Tolerance to serpentine substrate may involve multiple functional traits relating to morphology and ecophysiology, making the study of serpentine chaparral an ideal model system to enhance our understanding of the relative importance of adaptive evolution and ecological sorting processes in the evolution of plant communities.

During my post-doctoral tenure with Dr. David Ackerly (2003-2004), I conducted a comparative study of functional traits of serpentine tolerant shrub species and their serpentine intolerant relatives from several common chaparral lineages to address the following questions:

1. What are the key traits relating to functional morphology and ecophysiology that differ between the serpentine species and their closest relatives found on non-serpentine soil.

2. Do the findings support the hypothesis of convergent evolution (i.e., traits giving adaptation to serpentine have evolved independently in different lineages) or exaptation (i.e., traits were already present in the ancestors and ecological sorting processes have allowed the formation of serpentine chaparral) or both.

The preliminary phase of my research involved field measurements of functional traits of eight species pairs belonging to six plant families. Measures were taken twice a year, in the spring and late summer of 2003. The second phase will include common garden studies involving hydroponic and greenhouse experiments to characterize potentially adaptive traits relating to the tolerance to heavy metals (Ni, Cr), nutrients (N, Ca/Mg) and water stress, features that appear to be distinct between the various species pairs. See recent publication partially resulting from data generated from the study.

Lichens of Serpentine Barrens and Cinnabar Mines of Clear Creek Management Area, San Benito County, CA. See Recent Publication.